The circlatory system

Arteries : These carry blood away from the heart to tissues in the body. Arteries have thick walls composed of mainly smooth muscle and elastic tissue. The elastin allows the artey to expand to withstand the high pressure of blood. Arteries have a small lumen that helps to maintain high pressure of blood.

Arterioles : These branch off from arteries. Composed mainly of smooth muscle which helps to regulate blood flow to the capillary beds.

Capillaries : Capillaries are where the transported substances actually enter and leave the blood. Capillaries have a very narrow lumen and their walls are composed of a single layer of endothelium cells with gaps between them which makes capillaries very permeable. There are millions of them in the body which forms a huge surface area: volume ratio, which maximises the rate of exchange of substances.

Veins and Venules : Veins carry blood from tissues back to the heart. The smallest veins are called venules and they collect blood directly from the capillary beds. Venules and veins have thin inelastic walls made mostly of collagen fibres. Since the blood has lost almost all its pressure in the capillaries, veins and venules a large lumen to reduce the resistance to flow. They also have semi-lunar valves to prevent backflow of blood.